This invention relates generally to the field of magnetic data storage devices, and more particularly, but not by way of limitation, to a method of optimizing reader to writer offset compensation for a disc drive.
Disc drives are used for data storage in modem electronic products ranging from digital cameras to computer systems and networks. A typical disc drive includes a head-disc assembly (HDA), housing the mechanical portion of the drive, and a printed circuit board assembly (PCB) attached to the HDA, for controlling operations of the HDA and providing a communication link between the disc drive and its host.
Typically, an HDA comprises a recording surface affixed to a spindle motor assembly for rotation at a constant speed and an actuator assembly positionably controlled by a closed loop servo system. The actuator assembly supports a read/write head that traverse a plurality of radially spaced, generally concentric magnetic tracks on the recording surface. Disc drives using magneto resistive read/write heads typically use an inductive element, or writer, of the read/write head to write data to the magnetic tracks and a magneto resistive element, or reader, to read data from the magnetic tracks during drive operations. One type of data recorded to and read from the magnetic tracks is servo data. Servo data, including a physical track identification portion (also referred to as a servo track number or physical track number), written to the recording surface define each specific physical track of a number of physical tracks written on the recording surface. A servo track writer typically writes a predetermined number of physical tracks, also referred to as servo tracks, to each recording surface during the manufacturing process. The physical tracks are used by the closed loop servo system for controlling the position of the read/write head relative to the recording surface during disc drive operations.
High performance disc drives of the present generation typically achieve aerial bit densities measured in several gigabits per square centimeter, Gbits/cm2. Higher recording densities can be achieved by increasing the number of bits stored along each magnetic track or bits per inch (BPI), and/or by increasing the number of magnetic tracks provided per unit width or tracks per inch (TPI) across each recording surface. Increased BPI generally requires improvements in the read/write channel electronics to enable the data to be written (and subsequently read) at a correspondingly higher frequency.
Higher TPI generally requires improvements in servo control systems to enable the read/write heads to be more precisely positioned relative to the magnetic tracks. With increasing TPIs, separation between the reader and the writer of a fixed geometry MR read/write head, expressed as a number of magnetic tracks separating the two, increases at the same rate as the growth in TPI. As widths of the magnetic track decrease, the need to more precisely position the reader of the read/write head at track center of the magnetic track being read escalates. The heightened need to more precisely position the reader arises because the amplitude of the read signal drops off more sharply with each increment of lateral separation between the read element tracking at track center and data written off track center. The term track center is defined as the center of the servo data recorded on the magnetic track being read.
Because the reader is both laterally and longitudinally offset from the writer, and because the read/write head scribes an arch across the plurality of generally concentric magnetic tracks of the recording surface, the relative position of the reader in relation to track center of the magnetic tracks changes with the physical location of the read/write head relative to the recording surface. Near an outer diameter of the recording surface, the offset of the writer relative to track center of a magnetic track near the outer diameter of the recording surface and selected for writing data to, is at a first maximum. While near the inner diameter of the recording surface, the offset of the writer relative to track center of a magnetic track selected for writing data to, is at a second maximum. Near the center of the recording surface, the offset of the writer relative to track center of a magnetic track selected for writing data to, is substantially zero. The direction of offset of the second maximum offset from its track center is opposite from the direction of offset of the first maximum offset from its track center. Depending on the direction of offset, the compensation applied to the position signal during track following will be added as either a positive offset or a negative offset.
Typically, a plurality of logical tracks, each including a logical track identification portion (also referred to as a data track number or logical track number) is assigned and recorded onto the recording surface during a drive testing portion of the manufacturing process. Each logical track is associated with one of the physical tracks. During the servo write process, the physical location of each of the plurality of physical tracks, i.e., track spacing for the magnetic tracks and the physical track identification portions are defined and assigned to each magnetic track of the recording surface. Generally, a data track number does not correspond in value to a servo track number assigned the magnetic track. For example, data track xe2x80x9c0xe2x80x9d may be assigned, positioned and logical block addresses written to a magnetic track that coincides with servo track xe2x80x9c1450xe2x80x9d written to the recording surface during the servo write process.
Once each logical track has been assigned to one of the plurality of physical tracks, either a look-up table linking the relationship between a value of the logical track identification portion and a value of its associated physical track identification portion is provided, or a value to be used in translating between the value of the logical track identification portion and the value of its associated physical track identification portion is provided. During disc drive operations either the look-up table, the value or a conversion algorithm is used by a controller of the disc drive to convert from a requested logical track number to a corresponding servo track number, which the servo system uses to position the read/write head relative to the requested logical track.
During the process of assigning logical track numbers to each magnetic track, a calibration process is executed on selected logical tracks to determine an amount of compensation to be added by the servo system to position the writer to track center of the servo track associated with the selected logical track. Aligning maximum amplitude of the data being written to the selected logical track with track center of the servo track associated with selected logical track facilitates maximum recovery of the data during readback operation of the disc drive. The value of the compensation, measured in steps of the actuator, varies across the recording surface as a function of the diameter of the recording surface and an angle of presentation of the reader to the recording surface, which changes from the OD to the ID, across the recording surface.
The calibration process entails servoing on a physical track associated with a selected logical track; writing data to the selected logical track; stepping the reader over a half track to one side of the physical track and recording an amplitude of the data written to the logical track at that position; stepping the reader over a half track to the other side of the physical track and recording an amplitude of the data written logical track at that position; then collecting and analyzing amplitude measurements between those two positions to determine a maximum amplitude for the data written to the logical track; and then recording the number of steps and direction from track center needed to compensate the offset into a compensation table, also referred to as a MR jog table, for use by the servo system the disc drive executes write operations on the selected logical track. However, in the prior art, the MR jog table is set up based on physical track numbers, while the compensation values written to the table are based on logical track numbers. This disparity in track number identification leads to incorrect compensation values being accessed by the servo system.
For example, assume logical track xe2x80x9c10xe2x80x9d is selected for calibration, and logical track xe2x80x9c10xe2x80x9d corresponds to physical track xe2x80x9c160xe2x80x9d. Upon completing the calibration process for logical track xe2x80x9c10xe2x80x9d, the compensation value based on the measured offset of logical track xe2x80x9c10xe2x80x9d is linked in the MR jog table to physical track xe2x80x9c10xe2x80x9d. So the compensation value selected from the MR jog table associated with physical track xe2x80x9c10xe2x80x9d is the compensation value applicable to physical track xe2x80x9c160xe2x80x9d. For the present example, when a request is received by the servo controller to access logical track xe2x80x9c10xe2x80x9d, the servo controller converts the logical track xe2x80x9c10xe2x80x9d to physical track xe2x80x9c160xe2x80x9d. Next, the MR jog table is accessed for the offset value logged for physical track xe2x80x9c160xe2x80x9d, while the compensation value applicable for use with logical track xe2x80x9c10xe2x80x9d is linked to physical track xe2x80x9c10xe2x80x9d within the MR jog table. In this example, the compensation value read from the MR jog table associated with physical track xe2x80x9c160xe2x80x9d are applicable for use with logical track xe2x80x9c160xe2x80x9d. But when a request is received to position the writer in alignment with logical track xe2x80x9c160xe2x80x9d, the servo controller converts the requested logical track xe2x80x9c160xe2x80x9d to physical track xe2x80x9c310xe2x80x9d. Then the compensation value linked to physical track xe2x80x9c310xe2x80x9d is selected for application, which of course is an incorrect compensation value for logical track xe2x80x9c160xe2x80x9d. In a prior art disc drive, this mismatch in the application of compensation values posed few difficulties, because the resulting error fell below the ability of measuring the error.
The total number of tracks selected for use in determining compensation values is typically in the order of 50 to 100. The logical tracks selected for measurement are generally physically evenly distributed, from the OD to the ID, across the recording surface. The physical position, as opposed to the logical position, of the read/write head relative to the recording surface, has a direct bearing on the final amount of compensation needed.
Again, in prior art, as long as the difference between, for example, logical track 500 and servo track 500 was relatively small (50 to 200 tracks), the error created by selecting a compensation value based on the servo track number to compensate the position of the writer of the read/write head relative to the logical track number was minor. However, as TPI increases and the difference between a logical track number and its associated servo track number increases, the effects from the application of incorrect offset compensation values greatly affects the data recovery performance of high performance disc drives of the present generation.
High performance disc drives of the present generation typically have a difference between a logical track number and its associated servo track number of greater than 1,000 tracks.
The error created by applying incorrect compensation values is substantially a magnitude difference in raw bit error rate, when the difference between the servo track number and the logical track number is in the range of greater than 1,000 tracks. An increasing raw bit error rate leads directly to data throughput degradation thereby hindering the overall performance of the disc drive.
Therefore, challenges remain and a need persists for methods of optimizing reader to writer offset compensation for a disc drive using MR read/write heads. It is to this and other features and advantages set forth herein that embodiments of the present invention are directed.
The present invention provides a method for optimizing reader to writer offset compensation values for use by a disc drive. In accordance with one embodiment, the method includes steps of associating each logical track of a plurality of logical tracks to a physical track of a plurality of physical tracks, selecting a set of sample tracks from the plurality of physical tracks for use in determining offset compensation values.
Next, the method includes a step of determining an offset compensation value for each sample track of the set of sample tracks associated with one of the logical tracks is accomplished by measuring an offset value for each logical track associated with one of the sample tracks and deriving an offset compensation value for each of the sample tracks absent an association with one of the logical tracks.
Then, each measured offset compensation value and each estimated offset compensation value is linked with its associated physical track, followed by a step of calculating an offset compensation value for use with each of the physical tracks absent an association with one of the sample tracks. Collectively the measured, estimated and calculated compensation values provide optimized compensation values for use by the disc drive in aligning a writer of a read/write head to a track center of a selected physical track in preparation for writing data to a logical track associated with the selected physical track.
These and various other features and advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.